With the increasing popularity of the Global Positioning System (hereinafter “GPS”) finding the absolute position of an object above ground is now a relatively easy task. By combining a GPS receiver and a cellular phone transceiver and mounting the resulting device to a vehicle, it is also relatively easy to supply the position of the vehicle to a central location when the vehicle is reported as being stolen.
Accordingly, the prior art is replete with documents describing such systems and improvements to such systems. U.S. patent application Ser. No.2002/0115436 A1 entitled “Telematic System” and published on Aug. 22, 2002; and U.S. Pat. No. 5,986,543 entitled “Programmable vehicle monitoring and security system having multiple access verification devices” issued on Nov. 16, 1999 are good examples of such documents.
Since these methods and systems aim at tracking vehicles, it is possible to position the GPS receiver antenna so that it can receive the faint GPS signals used to compute the absolute position of the vehicle. However, such systems may not be used to track a device concealed in cargo since cargo is usually contained in enclosed spaces, such as trailers or warehouses, thereby further attenuating the already faint GPS signals.